Development Length Criteria: Bars Not Confined by Transverse Reinforcement

Abstract

An expression that accurately represents development and splice strength as a function of concrete cover and bar spacing is developed. The expression is then used to establish and evaluate modifications to the bond and development provisions of the ACI Building Code (AVI 318-89) for bars in concrete members that are not confined by transverse reinforcement. The expression for development and splice strength is similar in form to expression for development and splice strength is similar in form to expressions developed by Orangun, Jirsa, Breen (1975, 1977) but is obtained using techniques that limit the effects of unintentional bias in the test data. The resulting expression provides a more accurate representation of development and splice strength than do the earlier expressions, and it provides better guidance when there is a significant difference between the concrete cover and one-half of the clear spacing between bars. Proposals for new design cirteria, including one under study by ACI Committee 318, are compared. Each of the new proposals contains design criteria that are superior to the current development length provisions (ACI 318089); however, the criteria differ in terms of relative safety, economy, and ease of application. Side-by-side comparisons in design offices are recommended. In All cases, an additional development length modification factor of 1.1 is recommended for reinforcing steels with specified yield strengths in excess of 60,000 psi (414 MPa).